Lignin dissolution in dialkylimidazolium-based ionic liquid–water mixtures

Lignin dissolution in dialkylimidazolium-based ionic liquid (IL)–water mixtures (40 wt%–100 wt% IL content) at 60 °C was investigated. The IL content and type are found to considerably affect lignin solubility. For the IL–water mixtures except 1-butyl-3-methylimidazolium tetrafluoroborate ([C4C1im]BF4), the maximum lignin solubility can be achieved at 70 wt% IL content. Lignin solubility in IL–water mixtures with different cations follows the order 1-butyl-3-methylimidazolium ([C4C1im]+) > 1-hexyl-3-methylimidazolium ([C6C1im]+) > 1-ethyl-3-methylimidazolium ([C2C1im]+) > 1-octyl-3-methylimidazolium ([C8C1im]+) > 1-butyl-3-ethylimidazolium ([C4C2im]+) > 1-butyl-3-propylimidazolium ([C4C3im]+). For IL mixtures with different anions, lignin solubility decreases in the following order: methanesulfonate (MeSO3−) > acetate (MeCO2−) > bromide (Br−) > dibutylphosphate (DBP−). Evaluation using the theory of Hansen solubility parameter (HSP) is consistent with the experimental results, suggesting that HSP can aid in finding the appropriate range of IL content for IL–water mixtures. However, HSP cannot be used to evaluate the effect of IL type on lignin solubility.